1. Field of the Invention
The invention relates to an optical aperture device used in the manufacture of color cathode ray tubes (CRTs) of the dot matrix type.
2. Discussion of the Related Art
The display screen for such a dot matrix CRT comprises a large number of phosphor dots, usually embedded in or surrounded by a matrix of black, non-luminescent material. This matrix, as well as the phosphor dots, are deposited on the transparent face plate by well-known photolithographic processes, with an apertured shadow mask serving as the photo-stencil. The same mask is later mated with the screen during final assembly of the tube, in order to provide the closest registration between the location of the phosphor dots, produced by light beams, and the location of electron beam landings which illuminate the phosphor dots in the working CRT. The precise location of the dots on the screen is of great importance, as it determines the color purity and brightness uniformity of the finished tube. In modern high-resolution computer display tubes, dot location tolerances on the order of 0.0005 inches or less are common.
The light source for the photolithographic process usually takes the form of a high pressure mercury arc lamp. The length of the light-producing arc in such a lamp is normally much greater than its width; therefore a slotted aperture device, or diaphragm, closely adjacent to the lamp and oriented perpendicularly to the axis of the lamp is often used to produce an effective light source of approximately square cross section.
In a finished tube, the magnetic field produced by the deflection yoke bends the trajectories of the electrons emitted by each of the three electron guns and thus distributes them across the screen. However, if the trajectories of the electrons arriving at the screen from one particular gun are extended or extrapolated backwards, they do not appear to come from a single point; therefore, these trajectories cannot be simulated by light rays diverging from a single point source. For this reason, during the photolithographic exposure an aspherical correction lens is normally inserted between the light source and the shadow mask. Such a lens can be tailored, for example by a process of successive approximation, to modify the pattern of light rays so that it closely matches the pattern of electron trajectories mentioned above. For tubes of low to moderate resolution the resulting match may be good enough.
It has long been known, however, that the aspherical lens cannot provide a perfect match, or registration, between phosphor dot placement and electron beam landing, and means have been sought to correct the remaining errors, hereinafter called "beam landing errors." U.S. Pat. No. 3,780,629 to Barten and Ferguson teaches the use of a diaphragm having an aperture in the form of a crescent moon or horseshoe-shaped slot, inserted between an elongated arc lamp and the aspherical correction lens. The diaphragm lies either in a plane parallel to the axis of the light source, or in a cylindrical surface parallel to that axis. In a preferred embodiment, the cylindrical surface is concentric with the light source.
In the Barten-Ferguson device, the slot in the diaphragm is not closely adjacent to the elongated light source but is spaced therefrom. As a consequence, when the light source is viewed from different points on the screen, different portions of the lamp become visible through the slot. It might be said that the apparent point source is displaced when viewed from different points on the screen. This displacement, controlled by the shape of the slot and by its spacing from the light source, provides an extra variable which may be used to correct for some beam landing errors.
In the years since the Barten and Ferguson patent issued, the use of self-convergent deflection yokes for CRTs has become nearly universal. In this type of yoke, the magnetic deflection fields are intentionally made non-uniform. A pincushion-shaped field is used for horizontal deflection and a barrel-shaped field for vertical deflection. In addition, field shape varies along the yoke axis from the gun side to the screen side; for example, to minimize raster distortion, the vertical deflection field may change from barrel shape on the gun side to pin-cushion shape on the screen side. Electrons, during their travel through such a field, are subjected to transverse forces whose direction varies from point to point. The resulting twisting of the electron beam trajectories cannot be simulated by an optical lens having continuous (i.e. unbroken) surfaces. Thus, the resultant registration errors, sometimes referred to as "curl errors", have gone largely uncorrected in tubes that are screened with continuous lenses.
FIG. 1 illustrates the type of residual beam-landing errors observed on a tube using a self-convergent yoke. Only the centrally positioned "green" electron gun was turned on when the data was taken. The correction lens employed in making the screen had been designed to reduce the mean square beam landing error to a minimum. In the figure, each arrow represents the additional correction required, i.e., how far and in what direction the phosphor dots in that particular portion of the screen should be moved for perfect registration between light beam and electron beam landing. Conspicuous features of FIG. 1 are the swirl patterns and their apparent fourfold symmetry, i.e., an antisymmetric matrix form. With proper orientation of the yoke the symmetry axes correspond to the horizontal and vertical center lines of the screen. The Barton-Ferguson device cannot correct for this type of error distribution which is largely the result of the self converging yoke.
In the following discussions and figures, the axes will have the following designations: the X-axis represents the major axis of a rectangular CRT screen and the x-axis is parallel to it, but lies in the plane of the light source used to make the screen. The Y-axis represents the minor axis of the screen, and the y-axis is parallel to the Y-axis, but again lies in the plane of the light source. The z-axis is perpendicular to the screen, passes through the light source, and represents the axis of the finished tube.
It is convenient to consider the individual arrows in FIG. 1 as vectors F having two components FX and FY. Each of these components is a function of the screen coordinates X and Y. There are 9 rows of 11 vectors each in FIG. 1; together, these 99 vectors form a vector field.
Vector analysis defines a vector, curl F, whose magnitude, in the case of two dimensions, equals the difference between the partial derivatives of FY with respect to X and FX with respect to Y. The value of curl F corresponding to the vector field shown in FIG. 1 is plotted in FIG. 2, with circles indicating counterclockwise rotation and hexagons indicating clockwise rotation; the magnitude of rotation is proportional to the diameter of the circles or hexagons.
It can be shown that a lens with continuous surfaces cannot correct for those portions of the beam landing error F (X,Y) which produce finite values of curl F. Segmented lenses can correct for these curl errors but are very expensive to make.
It is, therefore, an object of this invention to provide means for minimizing the value of curl F, where F is a vector representing the beam landing error, and to do so throughout the range of screen coordinates X and Y.
It is a further object of the invention to provide means for minimizing the value of curl F, said means being constructed and arranged to take advantage of the high degree of symmetry in the distribution of curl F across the screen encountered when a self-convergent yoke is used for beam deflection.
It is also an object of the invention to provide photolithographic exposure apparatus in which an apertured member carrying a slot is inserted between an elongated light source and a correction lens, the center line of said slot lying in a plane which: 1) is perpendicular to said light source, and 2) contains an axis of symmetry of the uncorrected distribution of curl F. This axis of symmetry is usually, although not necessarily, the X or Y axis of the screen, as described above.